Though I don't think the difference is always clear-cut, I think I'd still tend to go with "couldn't" to refer to a general ability (or in this case, inability). - He couldn't move the sofa. He's not strong enough to do it without some help.
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AnonymousWould you say there is no difference ...I can't possibly think of all the sentences that might be possible with those words, so no, I wouldn't say there is no difference. There may be a difference I'm not aware of. Maybe it's just that I can't think of a counterexample at the moment. In general, though, I don't sense a great deal of differ